arXiv subjects

L. Balicas

Publications and source records attributed to L. Balicas.

At least 55 records · Page 3Linked to original sources

arXiv2013

Bulk evidence for a time reversal symmetry broken superconducting state in URu2Si2

URu$_2$Si$_2$ is claimed to be a chiral \emph{d}-wave superconductor with a $k_z (k_x \pm ik_y)$ time-reversal symmetry broken orbital component for the Cooper pair wave-function, which contains both nodal points and lines of nodes \cite{kasahara, kasahara2}. To study the magnetic response of such an unconventional state through a bulk, thermodynamic probe, we measured the magnetic torque $τ$ in very high-quality, well-characterized URu$_2$Si$_2$ single-crystals \cite{altarawneh,altarawneh2} at high magnetic-fields $H$ and at very low temperatures $T$. The magnetization $M(H) \propto τ(H) /H$ of URu$_2$Si$_2$, in its superconducting state and for angles within $15^{\circ}$ from the \emph{ab}-plane, reveals a change in its sign for $H$ approaching $H_{c2}$: from a clear diamagnetic response dominated by the pinning of vortices to a state with a smaller but "paramagnetic-like" hysteretic response which \emph{disappears} at $H_{c2}$, thus implying that it is intrinsically related to the superconducting state. We argue that this anomalous, angular-dependent behavior is evidence for a time-reversal symmetry broken superconducting state in URu$_2$Si$_2$, although not necessarily for the $k_z (k_x \pm ik_y)$ state.

cond-mat.supr-con↗

arXiv2013

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas van Alphen-effect

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which and a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin uctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase-diagram is indeed observed in the isostructural NaFeAs compound. Previous angle resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas van Alphen (dHvA) measurements which detected a few, electron like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results conciliate the apparent discrepancy between ARPES and the previous dHvA results5. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor a s-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP, suggest that the nodes in the gap function of LiFeP might be located on the hole-pockets.

cond-mat.supr-con↗

arXiv2013

Anomalous metallic state and anisotropic multiband superconductivity in Nb3Pd0.7Se7

We report the discovery of superconductivity in Nb$_3$Pd$_{x}$Se$_7$ with a $x$-dependent superconducting transition-temperature as high as $T_c \simeq 2.1 $ K for $x \simeq0.7$ (middle point of the resistive transition). Needle-like single crystals display anisotropic upper-critical fields with an anisotropy $γ= H^{b}_{c2}/H^{a}_{c2}$ as large as 6 between fields applied along their needle axis (or $b-$axis) or along the $a-$axis. As for the Fe based superconductors $γ$ is temperature-dependent suggesting that Nb$_3$Pd$_{0.7}$Se$_7$ is a multi-band superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole- and electron-character. Remarkably, $H^{b}_{c2}$ is observed to saturate at $H^{b}_{c2}(T \rightarrow 0 \text{K}) \simeq 14.1$ T which is $4.26 \times H_p$ where $H_p$ is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb$_2$Pd$_{x}$Se$_5$ phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e. displaying $\partial ρ/ \partial T < 0$ above $T_c$ similarly to what is observed in the pseudogap-phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multi-band character, extremely high and anisotropic $H_{c2}$s, are all ingredients for unconventional superconductivity.

cond-mat.supr-con↗

arXiv2013

Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES

The Kondo insulator SmB6 has long been known to exhibit low temperature (T < 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the Gamma points of the surface Brillouin zone. These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. While the topological nature of the in-gap metallic states cannot be ascertained without spin (spin-texture) measurements our bulk and surface measurements carried out in the transport-anomaly-temperature regime (T < 10K) are consistent with the first-principle predicted Fermi surface behavior of a topological Kondo insulator phase in this material.

cond-mat.str-el↗

arXiv2013

Intrinsic carrier mobility of multi-layered MoS$_2$ field-effect transistors on SiO$_2$

By fabricating and characterizing multi-layered MoS$_2$-based field-effect transistors (FETs) in a four terminal configuration, we demonstrate that the two terminal-configurations tend to underestimate the carrier mobility $μ$ due to the Schottky barriers at the contacts. For a back-gated two-terminal configuration we observe mobilities as high as 125 cm$^2$V$^{-1}$s$^{-1}$ which is considerably smaller than 306.5 cm$^2$V$^{-1}$s$^{-1}$ as extracted from the same device when using a four-terminal configuration. This indicates that the intrinsic mobility of MoS$_2$ on SiO$_2$ is significantly larger than the values previously reported, and provides a quantitative method to evaluate the charge transport through the contacts.

cond-mat.mes-hall↗

arXiv2013

Superconductivity with extremely large upper critical fields in Nb2Pd0.81S5

Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb2Pd0.81S5, with a transition temperature Tc > 6.6 K. Despite its relatively low Tc, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. mu_0 H_{c2} (T approaching 0 K). 37 T for fields applied along the crystallographic b-axis. For a field applied perpendicularly to the b-axis, mu_0 H_{c2} shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb2Pd0.81S5 is a multi-band superconductor. This is consistent with band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. The static spin susceptibility as calculated through the random phase approximation, reveals strong peaks suggesting proximity to a magnetic state and therefore the possibility of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2013

Vortex lock-in transition and evidence for transitions among commensurate kinked vortex configurations in single-layered Fe arsenides

We report an angle-dependent study of the magnetic torque $τ(θ)$ within the vortex state of single-crystalline LaO$_{0.9}$F$_{0.1}$FeAs and SmO$_{0.9}$F$_{0.1}$FeAs as a function of both temperature $T$ and magnetic field $H$. Sharp peaks are observed at a critical angle $θ_c$ at either side of $θ=90^{\circ}$, where $θ$ is the angle between $H$ and the inter-planar \emph{c}-axis. $θ_c$ is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of $θ$ and as $θ$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the inter-planar distance leading to additional kinked vortex configurations.

cond-mat.supr-con↗

arXiv2013

Anomalous hysteresis as an evidence for a magnetic field-induced chiral superconducting state in LiFeAs

Magnetometry measurements in high quality LiFeAs single-crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field $H_{c2}$, from a clear diamagnetic response dominated by the pinning of vortices, to a considerably smaller net hysteretic response of opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response at high fields results from pinned vortices and associated screening super-currents, this sign change must result from currents circulating in the opposite sense, which give rise to a small field-dependent magnetic moment \emph{below} $H_{c2}$. This behavior seems to be extremely sensitive to the sample quality or stoichiometry, as we have observed it only in a few fresh crystals, which also display the de Haas van Alphen-effect. We provide arguments against the surface superconductivity, the flux compression, and the random $π$ junction scenarios, which have been previously put forward to explain a paramagnetic Meissner effect, below the lower critical field $H_{c1}$. The observed anomalous hysteresis at high fields will be compatible with the existence of chiral gap wave-functions, which possess a field dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate how a $(d_{x^2 - y^2} + id_{xy})$ or a $(p_x+ip_y)$ chiral superconducting component can be stabilized in the mixed state of $s_{\pm}$ superconductor, due to the combined effects of the magnetic field and the presence of competing pairing channels. The realization of a particular chiral pairing depends on the microscopic details of the strengths of the competing pairing channels.

cond-mat.supr-con↗

arXiv2012

High superconducting anisotropy and weak vortex pinning in Co doped LaFeAsO

Here, we present an electrical transport study in single crystals of LaFe$_{0.92}$Co$_{0.08}$AsO ($T_c \simeq 9.1$ K) under high magnetic fields. In contrast to most of the previously reported Fe based superconductors, and despite its relatively low $T_c$, LaFe$_{1-x}$Co$_x$AsO shows a superconducting anisotropy which is comparable to those seen for instance in the cuprates or $γ_H = H_{c2}^{ab}/H_{c2}^{c} = m_c/m_{ab} \simeq 9$, where $m_c/m_{ab}$ is the effective mass anisotropy. Although, in the present case and as in all Fe based superconductors, $γ\rightarrow 1$ as $T \rightarrow 0$. Under the application of an external field, we also observe a remarkable broadening of the superconducting transition particularly for fields applied along the inter-planar direction. Both observations indicate that the low dimensionality of LaFe$_{1-x}$Co$_x$AsO is likely to lead to a more complex vortex phase-diagram when compared to the other Fe arsenides and consequently, to a pronounced dissipation associated with the movement of vortices in a possible vortex liquid phase. When compared to, for instance, F-doped compounds pertaining to same family, we obtain rather small activation energies for the motion of vortices. This suggests that the disorder introduced by doping LaFeAsO with F is more effective in pinning the vortices than alloying it with Co.

cond-mat.supr-con↗

arXiv2011

Disorder-dependent superconducting phase-diagram at high magnetic fields in Fe$_{1 + y}$Se$_{x}$Te$_{1-x}$ ($x \sim 0.4$)

We compare the superconducting phase-diagram under high magnetic fields (up to $H = 45$ T) of Fe$_{1+y}$Se$_{0.4}$Te$_{0.6}$ single crystals originally grown by the Bridgman-Stockbarger (BRST) technique, which were annealed to display narrow superconducting transitions and the optimal transition temperature $T_c \gtrsim 14$ K, with the diagram for samples of similar stoichiometry grown by the traveling-solvent floating-zone technique as well as with the phase-diagram reported for crystals grown by a self-flux method. We find that the so-annealed samples tend to display higher ratios $H_{c2}/T_c$, particularly for fields applied along the inter-planar direction, where the upper critical field $H_{c2}(T)$ exhibits a pronounced downward curvature followed by saturation at lower temperatures $T$. This last observation is consistent with previous studies indicating that this system is Pauli limited. An analysis of our $H_{c2}(T)$ data using a multiband theory suggests the emergence of the Farrel-Fulde-Larkin-Ovchnikov state at low temperatures. A detailed structural x-ray analysis, reveals no impurity phases but an appreciable degree of mosaicity in as-grown BRST single-crystals which remains unaffected by the annealing process. Energy-dispersive x-ray analysis showed that the annealed samples have a more homogeneous stoichiometric distribution of both Fe and Se with virtually the same content of interstitial Fe as the non-annealed ones. Thus, we conclude that stoichiometric disorder, in contrast to structural disorder, is detrimental to the superconducting phase diagram of this series under high magnetic fields. Finally, a scaling analysis of the fluctuation conductivity in the superconducting critical regime, suggests that the superconducting fluctuations have a two-dimensional character in this system.

cond-mat.supr-con↗

arXiv2011

High pressure sequence of Ba_3NiSb_2O_9 structural phases: new $S = 1$ quantum spin-liquids based on Ni^{2+}

By using a high pressure, high temperature (HPHT) technique, the antiferromagnetically ordered ($T_N$ = 13.5 K) 6H-A phase of Ba$_3$NiSb$_2$O$_9$ was transformed into two new gapless quantum spin liquid(QSL) candidates with S=1 (Ni$^{2+}$) moments: the 6H-B phase with a Ni$^{2+}$-triangular lattice and the 3C-phase with a Ni$^{2+}$-three-dimensional (3D) edge-shared tetrahedral lattice. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures $θ_{CW}$ of -75.5 K (6H-B) and -182.5 K (3C), respectively. Below $\sim 25$ K the magnetic susceptibility of the 6H-B phase saturates to a constant value $χ_0 = 0.013 $ emu/mol which is followed below 7 K, by a linear-temperature dependent magnetic specific heat ($C_M$) displaying a giant coefficient $γ$ = 168 mJ/mol-K$^2$. Both observations suggest the development of a Fermi-liquid like ground state characterized by a Wilson ratio of 5.6 in this insulating material. For the 3C phase, the $C_M \propto T^2$ behavior indicates a unique S=1, 3D QSL ground-state.

cond-mat.str-el↗

arXiv2011

Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $τ(θ)$ in the mixed state of LaO$_{0.9}$F$_{0.1}$FeAs single crystals as a function of temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter $γ=(m_c/m_{ab})^{1/2}$ from $τ(θ)$. We show that $γ$ depends strongly on both $T$ and $H$, reaching a maximum value of $\sim$ 10 followed by a decrease towards values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth $λ_{ab}$ and $γ$ suggest the onset of suppression of a superconducing gap at $H \approx H_{c2}/3$.

cond-mat.supr-con↗

arXiv2011

Anisotropic hysteretic Hall-effect and magnetic control of chiral domains in the chiral spin states of Pr$_2$Ir$_2$O$_7$

We uncover a strong anisotropy in both the anomalous Hall effect (AHE) and the magnetoresistance of the chiral spin states of Pr$_2$Ir$_2$O$_7$. The AHE appearing below 1.5 K at zero magnetic field shows hysteresis which is most pronounced for fields cycled along the [111] direction. This hysteresis is compatible with the field-induced growth of domains composed by the 3-in 1-out spin states which remain coexisting with the 2-in 2-out spin ice manifold once the field is removed. Only for fields applied along the [111] direction, we observe a large positive magnetoresistance and Shubnikov de Haas oscillations above a metamagnetic critical field. These observations suggest the reconstruction of the electronic structure of the conduction electrons by the field-induced spin-texture.

cond-mat.str-el↗

arXiv2011

Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements

The Hall coefficient R_H of the cuprate superconductor YBa2Cu3Oy was measured in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152, in the underdoped regime. In fields large enough to suppress superconductivity, R_H(T) is seen to go from positive at high temperature to negative at low temperature, for p > 0.08. This change of sign is attributed to the emergence of an electron pocket in the Fermi surface at low temperature. At p < 0.08, the normal-state R_H(T) remains positive at all temperatures, increasing monotonically as T \to 0. We attribute the change of behaviour across p = 0.08 to a Lifshitz transition, namely a change in Fermi-surface topology occurring at a critical concentration p_L = 0.08, where the electron pocket vanishes. The loss of the high-mobility electron pocket across p_L coincides with a ten-fold drop in the conductivity at low temperature, revealed in measurements of the electrical resistivity $ρ$ at high fields, showing that the so-called metal-insulator crossover of cuprates is in fact driven by a Lifshitz transition. It also coincides with a jump in the in-plane anisotropy of $ρ$, showing that without its electron pocket the Fermi surface must have strong two-fold in-plane anisotropy. These findings are consistent with a Fermi-surface reconstruction caused by a unidirectional spin-density wave or stripe order.

cond-mat.supr-con↗

arXiv2011

{Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsO$_{0.9}$F$_{0.1}$ single crystals

The low-temperature antiferromagnetic state of the Sm-ions in both nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed magnetic fields, as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the $ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in $35-40$~T is essentially temperature independent and points towards a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60~T. Such a large field value is particularly remarkable when compared to the relatively small Néel temperature $T_{\rm N}\simeq5$~K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field-dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This is indicates a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long range antiferromagnetic order of the Sm ions.

cond-mat.supr-con↗

arXiv2010

Fermi-surface of underdoped LaFeAsO1-xFx as determined by the Haas-van Alphen-effect

Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newly discovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of the Fermi surface associated with their antiferromagnetic and superconducting phases, which is essential for understanding their magnetism, pairing symmetry and superconducting mechanism. Calculations 4 and surface-sensitive measurements 5,6,7 provided early guidance, but lead to contradictory results, generating a need for a direct experimental probe of their bulk Fermi surface. In antiferromagnetic LaFeAsO1-xFx 8,9 we observe a complex pattern in the Fourier spectrum of the oscillatory component superimposed onto the magnetic torque signal revealing a reconstructed Fermi surface, whose geometry is not fully described by band structure calculations. Surprisingly, several of the same frequencies, or Fermi surface cross-sectional areas, are also observed in superconducting LaFeAsO1-xFx (with a superconducting transition temperature Tc ~ 15 K). Although one could attribute this to inhomogeneous F doping, the corresponding effective masses are largely enhanced with respect to those of the antiferromagnetic compound. Instead, this implies the microscopic coexistence of superconductivity and antiferromagnetism on the same Fermi surface in the underdoped region of the phase diagram of the LaFeAsO1-xFx series. Thus, the dHvA-effect reveals a more complex Fermi surface topography than that predicted by band structure calculations4 upon which the currently proposed superconducting pairing scenarios10,11,12,13 are based, which could be at the origin of their higher Tcs when compared to their phosphide analogs.

cond-mat.supr-con↗

arXiv2010

Anisotropic phase diagram of the frustrated spin dimer compound Ba3Mn2O8

Heat capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba3Mn2O8 in the field range from 0T to 18T. For fields oriented along the c axis a single magnetically ordered phase is found in this field range, whereas for fields oriented along the a axis two distinct phases are observed. The present measurements reveal a surprising non-monotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (Hc1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next nearest Mn ions, the Zeeman energy and single ion anisotropy. This Hamiltonian also predicts a non-monotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.

cond-mat.str-el↗

arXiv2010

Unusual metamagnetism in CeIrIn$_5$

We report a high field investigation (up to 45 T) of the metamagnetic transition in CeIrIn$_5$ with resistivity and de-Haas-van-Alphen (dHvA) effect measurements in the temperature range 0.03-1 K. As the magnetic field is increased the resistivity increases, reaches a maximum at the metamagnetic critical field, and falls precipitously for fields just above the transition, while the amplitude of all measurable dHvA frequencies are significantly attenuated near the metamagnetic critical field. However, the dHvA frequencies and cyclotron masses are not substantially altered by the transition. In the low field state, the resistivity is observed to increase toward low temperatures in a singular fashion, a behavior that is rapidly suppressed above the transition. Instead, in the high field state, the resistivity monotonically increases with temperature with a dependence that is more singular than the iconic Fermi-liquid, temperature-squared, behavior. Both the damping of the dHvA amplitudes and the increased resistivity near the metamagnetic critical field indicate an increased scattering rate for charge carriers consistent with critical fluctuation scattering in proximity to a phase transition. The dHvA amplitudes do not uniformly recover above the critical field, with some hole-like orbits being entirely suppressed at high fields. These changes, taken as a whole, suggest that the metamagnetic transition in CeIrIn$_5$ is associated with the polarization and localization of the heaviest of quasiparticles on the hole-like Fermi surface.

cond-mat.str-el↗